1. Field of the Invention
This invention relates to a nonaqueous electrolyte secondary cell, more particularly to an active anode substance of the nonaqueous electrolyte secondary cell and a production method thereof, and a nonaqueous electrolyte secondary cell incorporating the anode substance.
2. Description of the Prior Art
Recently with spread of portable appliances such as video cameras, demand for reusable secondary cells in place of conventional disposable cells is increasing. The majority of secondary cells now available in the market are nickel-cadmium cells involving alkali solutions. It has been difficult, however, to improve further the energy density of a conventional cell because its voltage is about 1.2V. It presents with another problem: its self discharge rate rises as high as 20% when left at room temperature for one month.
To meet these problems studies have been directed to a nonaqueous electrolyte secondary cell which incorporates a nonaqueous solvent as an electrolyte solution and a light metal like lithium as a material of the anode, thereby to raise the voltage up to 3V, to increase the energy density, and to reduce the self-discharge rate. With such a secondary cell, however, the metal lithium used as a material of the anode grows into dendritic processes under the influence of repeated charges/discharges, and the processes may come into contact with the cathode to form a shunt within the cell, which may cause the life of the cell to shorten. This problem hinders the cell from being put into practice.
To meet this problem, a nonaqueous electrolyte secondary cell which incorporates an alloy of lithium or the like with other metals as a material of the anode has been proposed. The same problem still holds for this cell: repeated charges/discharges turn the alloy into fine particles, which may lead to the shortened life of the cell.
To meet above problems, for example, as disclosed in Japanese Unexamined Patent Publication No. 62-90863, a nonaqueous electrolyte secondary cell has been proposed in which a carbon material like cokes is employed as an active anode substance. This secondary cell, being free from above problems, is excellent in endurance to cyclic activation. Further, as disclosed by the present authors in Japanese Unexamined Patent Publication No. 63-135099, when LixMO.sub.2 (where M represents one or more transition metals and x is a number not less than 0.05 and not more than 1.10) is used as an active cathode substance, the resulting nonaqueous electrolyte secondary cell will have a long life and a high energy density.
The nonaqueous electrolyte secondary cell which incorporates a carbon material as an active anode substance is more excellent in endurance to cyclic activation and safety as compared with the secondary cell which incorporates a metal lithium as an active anode substance, but is inferior in energy density to the latter. To improve this defect measures have been taken to improve the filling density, but it still remains a technically unsolved problem.
One of the reasons why this type of cell is inferior in energy density may lie in the fact that it requires the addition of a binder for adherence of a powder acting as an active substance. Namely, when a carbon material is used, a material like pitch is sintered and ground, or the material, after being ground, is sintered anew to produce a powder to be ready for use. Then, to this powder are added a binder such as rubber and a dispersant to produce a slurry. The slurry is applied on an electricity collecting body or is molded into pellets, to produce an anode. Thus, the electrode is composed of three elements; a carbon material, a binder and an electricity collecting body. The binder typically is added by 3-20%. The content of the active substance (filling density) in the cell is restricted by that amount of the binder, which will impose a limit on the capacity of the cell.